Due to their crystalline character, ceramics and glassceramics exhibit optical properties which make them particularly interesting relative to the corresponding glasses. In addition, applications in optics require the use of transparent or at least translucid ceramics, a prerequisite feature for good optical properties. The relevant wavelengths are those of the visible light spectrum, that is, between 400 nm and 800 nm.
However, if it is easy to produce transparent glasses, producing transparent or at least completely translucid (ceramics) or partially (glassceramics) crystallised materials is much more difficult.
Compositions of transparent or translucid crystallised materials have been described in the prior art, however they correspond essentially either to monocrystals or to nanocrystallised ceramics.
Manufacturing processes of monocrystals, often based on the Czochralski method, require manufacturing times ranging from several days to several weeks and operating temperatures greater than 1500° C. for usually used oxides. The resulting crystals have a size of the order of a few centimetres.
Manufacturing processes for nanocrystallised ceramics generally make use of nanometric particles, costly to make from precursors. The particles undergo a pressing step, then a sintering step at a temperature often greater than 1500° C. During this sintering step growth of the crystals occurs. Mastering this step is particularly critical, since it is in general necessary to conserve crystals of nanometric size, usually less than 100 nm (less than the lower limit of the visible wavelength), to retain transparency. If the crystalline growth is too important, opaque ceramics are obtained.
These monocrystals and nanocrystallised ceramics, highly adapted for high performance applications such as LASER applications, have a manufacturing cost far too high for commoner applications, for example for display, lighting or medical imaging.
There is therefore a need for novel ceramics and transparent or translucid glassceramics which combine good optical characteristics, such as luminescence for example, with a relatively inexpensive manufacturing process such as a glassmaking process followed by annealing of the glass.
U.S. Pat. No. 3,635,739 describes transparent or translucid glassceramics composed of 4 to 15% by weight of BaO, 35 to 45% by weight of CaO and 35 to 45% by weight of Al2O3, obtained by annealing a glass having the composition indicated earlier, comprising also from 5 to 15% by weight of oxide acting as nucleation catalyst, such as ZrO2 and/or V2O3 and/or of Ta2O3. These glassceramics are crystallised at more than 50% by weight, and generally more than 75% by weight.
It should be noted that the composition of these ceramics is such that the stoxhiometry in CaO is at least 2 times greater than the stcechiometry in BaO.
Contrary to U.S. Pat. No. 3,635,739 which presents glassceramics based on calcium aluminates, the applicant surprisingly found novel compositions based on barium and/or strontium alurninate, with a CaO content much less than that of barium/strontium, or even zero, for making transparent or translucid ceramics and glassceramics having crystals of micrometric size. It would seem in fact that ceramics and glassceramics according to the invention have isotropic optical characteristics, and/or that the difference in refraction index between the crystals and the vitreous phase is sufficiently low for the material to have good transparency properties.
The ceramics and glassceramics according to the invention can be easily obtained by means of an inexpensive glassmaking process, involving an annealing step of a glass of corresponding composition based on barium and/or strontium aluminate.